In the field of large area electronics, low cost but high performance sensors are one of the most targeted applications. These sensors can be used for a variety of acquisition functions such as thermal measurements, x-ray detection, and pressure sensing, to name a few. These devices are used in diverse fields including medical, environmental, security and industrial, amongst others. Development and commercialization of these sensing arrays are usually dictated by the cost of the processing technology used to build them as well as their sensing accuracy. Because these sensors are built over a large area, selecting a suitable technology that can yield the appropriate level of accuracy at an acceptable cost is often difficult.
The most efficient and accurate sensing arrays are based on active principles. Active sensors quantify a specific physical parameter response to a given stimulus. For example, active thermal sensors measure an object's heat conductance for a given heating stimulus. Examples of sensors of this type are disclosed in U.S. Pat. No. 6,091,837 to Dinh, entitled “Sensor for Acquiring a Fingerprint Image Based on Heat Transfer” (hereinafter “Dinh I”) and WO 2006/033582 A1, also to Dinh, entitled “Apparatus for Fingerprint Sensing and Other Measurements” (hereinafter, “Dinh II”), the entirety of each of which is hereby incorporated by reference herein. The response to the stimulus is measured by each of the sensing sites within a sensor array. The response is in part a function of the stimulus provided, i.e., the larger the stimulus, the larger the response.
Recent advances in lower cost semiconductor electronics, such as high performance polycrystalline silicon (“polysilicon”) thin film transistors (TFTs), have enabled the implementation of accurate sensing arrays at a reduced cost. Use of this technology also provides the ability to integrate control circuitry on the same panel as the sensing array, further reducing cost and increasing integration levels. One of the main drawbacks of using these lower cost technologies, however, is their limited device performance when compared to conventional, single crystalline electronics. Even though material properties have improved dramatically in recent years, improvements are still needed. For example, it is important to acquire an electric signal which is free of electrical noise (i.e., a signal that has good signal-to-noise ratio). This ratio eventually determines the accuracy of the system. Thin films transistor devices formed using polycrystalline material can generate high amounts of electrical noise particularly, when using a large stimulus to obtain a high response. This makes the task of obtaining a good signal to noise ratio difficult.
Therefore, sensing devices having improved device performance are desired.